Heat-sensitive recording materials are well-known, and utilize the color forming reaction between a leuco dye and a developer to produce recorded images by heat. Such heat-sensitive recording materials are relatively inexpensive, and the recording apparatuses are compact and easily maintained. For these reasons, heat-sensitive recording materials have found a wide range of uses, for example, they are used not only as recording media for the output of facsimiles and a variety of computers, printers of scientific measuring equipment, et cetera, but are also used as recording media for a variety of printers of POS labels, ATMs, CAD, handy terminals, paper for various tickets, et cetera.
However, when such heat-sensitive recording materials come into contact with oil, and plasticizers such as films or the like, alcohol, water, etc., problems occur such as fading of the color of recorded images, background coloration, adhesion of residue to the thermal-recording head during recording and the like.
In order to avoid such problems, a protective layer composed of, e.g., a water-soluble resin such as polyvinyl alcohol, starch, acrylic resin or the like and a pigment such as kaolin, calcium carbonate, amorphous silica, colloidal silica or the like (see for example, Japanese Unexamined Patent Publication Nos. 1993-147354; 1995-9762; 2000-118138; 2000-238432; 2002-240430; 2004-223994; and 2003-191647), was placed on the heat-sensitive recording layer. Pigments such as calcium carbonate, amorphous silica and colloidal silica have been used for preventing the adhesion of residue to the thermal head. For example, a heat-sensitive recording material comprising a protective layer principally composed of a resin and a filler with a Mohs hardness of 2.0 or less has been proposed which does not cause thermal-head wear and has less adhesion of residue to the thermal head (see for example Japanese Unexamined Patent Publication No. 1993-147354).
In recent years, heat-sensitive recording materials have frequently been used as printed ticket forms and similar forms and documents. Printing with ultraviolet curable ink, in particular, has been widely used, since it offers advantages such as: (1) solvent-free, therefore safety guaranteed; (2) fast drying speed due to ultraviolet drying; (3) energy savings achieved by compact UV irradiators; and (4) lowered drying temperature that leads to less background fogging by heat, especially in heat-sensitive recording materials.
However, protective layers with satisfactory properties have yet to be attained, because conventional protective layers present problems such as (a) low adhesion of ink to heat-sensitive recording materials causes printed surfaces to be easily removed by, for example, cellophane tape; (b) during recording with a thermal head, ink fuses by heat and adheres to the thermal head, easily causing a sticking phenomenon; and (c) the thickness of an ink layer printed on the surface of the protective layer of a heat-sensitive recording layer attenuates the recording energy from the thermal head, resulting in lowered recording sensitivity.
Therefore, there is a need to improve the surface strength of the press print ink and water resistance to heat sensitive recording material.